c.1400, from Old French trépaner (14c.), from trepan (n.), the name of the tool with which it was done, from Medieval Latin trepanum "a saw for cutting out small pieces of bone from the skull," from Greek trypanon "a borer, an auger, a carpenter's tool; a surgeon's trepan," from trypan "to bore," related to trype "hole" (cognate with Old Church Slavonic truplu "hollow"), from PIE *trup-, from root *tere- (1) "to rub, turn" (see throw (v.)). Related: Trepanned; trepanning.